An ohmmeter is an instrument for measuring the resistance of a conductor directly in ohms. Typically, measurement of a DC resistance is accomplished by applying a known current to an unknown resistance and measuring the voltage developed across the resistance. The value of the unknown resistance may then be calculated using Ohm's law.
Ohmmeters are known for testing an unknown resistance across input terminals where a "live" voltage potential exists. These types of ohmmeters are known as in-circuit ohmmeters. Such in-circuit ohmmeters, however, may be damaged if the voltage at the input terminals is sufficiently great (e.g. on the order of one or two kilovolts (KV)). Moreover, because typical ohmmeter circuits include only a single current source for generating the current which is applied to the unknown resistance, the current source may be damaged if the polarity of the live voltage encountered at the terminals is opposite that which the current source can handle.
Conventionally, high voltage relays are used to switch the ohmmeter input terminals into and out of the circuit(s) to be tested so as not to damage the ohmmeter. If an excessive voltage is encountered which may damage the ohmmeter, the high voltage relay circuit senses the excessive voltage and electromechanically disconnects the ohmmeter from the circuit under test. The relay connects the ohmmeter to the circuit during intervals of time in which no excessive voltage will be applied to the ohmmeter input terminals.
The additional requirement of high voltage relay in a typical in-circuit ohmmeter, however, requires additional ohmmeter circuit components in the form of the high voltage relay. Moreover, this additional relay adds operational time to the ohmmeter circuit, as typical high voltage relays take on the order of between 20-30 milliseconds (ms) to switch the ohmmeter into and out of the circuit under test. Moreover, electromechanical disconnect relays are subject to inconsistent operation and failure.
Accordingly it is an object of the present invention to provide an accurate and reliable in-circuit ohmmeter which does not utilize electromechanical relays, which provides protection against high voltage encountered at its input terminals, and which permits rapid in-circuit operation regardless of the polarity of voltage experienced at the input terminals.